【Objective】Straw returning is an effective measure to increase the amount of soil organic carbon (SOC) in agroecosystems. The objective of this study was to study the temporal dynamic of SOC in Mollisols during long-term straw return. 【Method】Based on an 18-year long-term field experiment using a maize-soybean rotation cropping system in the typical black soil region，the temporal dynamic of topsoil (0-20 cm) organic carbon was studied under three treatments: no fertilizer (NF), mineral fertilizer (NPK), and mineral fertilizer with straw return (NPKS). Furthermore, physical and chemical fraction methods were applied to study the effect of straw return on SOC pools. 【Result】The results showed that: (1) Compared with the initial (2004) soil, the SOC content significantly increased by 12.97% in the NPKS treatment, with an annual increase of 0.18 g·kg-1, and the SOC content significantly decreased by 3.9% in the NF treatment, while no significant change was found in NPK treatment. (2) There was a significant positive correlation between SOC content and year and the cumulative carbon input in the NPKS treatment. In particular, a significant relationship between SOC and cumulative carbon was observed from 2004 to 2015 under NPKS, while not from 2015 to 2022, indicating that the increase of SOC caused by straw return mainly occurred in the first 11 years, and after 11 years, the SOC reached a state of equilibrium. (3) The NPKS treatment increased the carbon content in free light fraction (fLFC), occluded light fraction (oLFC), the heavy fraction (HFC), humic acid (HAC), fulvic acid (FAC), and humin (HMC) by 47.77%, 34.77%, 11.18%, 13.00%, 6.32%, and 11.71%, respectively. Straw return improved the C proportion in labile fractions (fLFC and oLFC) and decreased the proportion of HFC, but the contribution of HFC to SOC improvement was more than 80%. Thus, the stable HFC was a key component for the long-term sequestration of SOC. In addition, straw return increased the ratio of HA/FA, consequently, increasing the humification degree of soil organic matter. 【Conclusion】Long-term continuous straw return can effectively improve the content of SOC and its fractions in black soil, but after 11 years of continuous straw return, SOC will stop growing and reach a new equilibrium. Although the labile SOC was elevated at a higher percentage than the stable SOC，the stable SOC still plays a crucial role in maintaining the stability and quantity of SOC.